The present invention relates to mixed-signal circuits, and more particularly to programmable active filters that improves the bandwidth resolution, wherein the programmability can be performed either by the user or by the manufacturer by means of a look-up table.
In mixed-signal circuits, a common requirement is that the bandwidth of an analog frequency-selective filter be programmed to a desired value dependent on several variables, such as bandwidth mode and process correction factor. Furthermore, gain selection may affect time constants requiring a further correction.
When implementing integrated programmable active filters, processing and temperature variations can cause values of the components to vary, which in turn cause the circuit parameters such as the RC product to vary. One approach may use operational amplifiers having high-precision external resistors and capacitors. Another approach may use an integrated filter having an RC product that can be programmed to compensate for process and temperature variations.
FIG. 1A shows a schematic of a low-pass filter 10 whose cut-off frequency fc is shown as being dependent on the gain, bandwidth and processing parameters. FIG. 1B is a conventional implementation of low-pass filter 10. Low-pass filter 10 includes an operational amplifier (op-amp) 11 having a differential input x(t) and a differential output y(t). Op-amp 11 may be a gain circuit having two inverting feedback paths 12, 13. An inverting output is coupled to a non-inverting input of op-amp 11 via feedback path 12, and a non-inverting output is coupled to an inverting input of the op-amp via feedback path 13. In the example shown, the low-pass filter is a differential topology to reject common mode noise. It is appreciated that the filter can be converted to a single-ended topology by connecting either of the signal inputs to ground. In the following, only one feedback path will be described as the other feedback path has a mirror function and does not change the gain, the bandwidth mode and the process variation of the filter.
Feedback path 12 includes a resistor Rfa that together with input resistor Rp defines the gain of the low-pass filter. Feedback path 12 also includes a capacitor C1a that together with resistor Rfa defines the 3 dB cut-off frequency (i.e., the bandwidth) of the low-pass filter. By adjusting the value of resistor Rfa and/or capacitor C1a, the bandwidth of the low-pass filter can be controlled or corrected, e.g., to compensate for process and temperature variations or to adapt to application demands. In the example described above, the negative input is assumed to be connected to ground, so that the output y(t) is inverting. Similarly, the low-pass filter 10 can also be described as a single-ended low-pass filter having feedback path 13 interposed between the positive output and the negative input of the operational amplifier 11 and the positive input connected to ground. In this case, the gain of the low-pass filter is defined as the ration ratio of Rfb/Rn and the cut-off frequency is defined as fc=1/(2π*Rfb*C1b).
As shown in FIG. 1B, low-pass filter 10 can be tuned by using a tunable capacitor arrays and multiple resistors connected in series and/or in parallel. In general, the value of capacitance and resistance per unit area in integrated circuit manufacturing processes is not well controlled, and in order to have an appropriate bandwidth resolution, a large number of capacitors and consequently a large number of control bits are required. The large number of capacitors may require a large layout area and increase the cost of the programmable filters.
Accordingly, it is desirable to provide a user or manufacturer-programmable capacitor value using a small number of capacitor values and a small number of control bits for use in programmable filters.